Field of Invention
This invention relates to high isolation power combiners/splitters and couplers, specifically to such combiners/splitters and couplers that are used in RF and microwave circuits.
Description of Prior Art
Special circuits are sometimes required to combine two or more high frequency RF or microwave signals. A commonly used circuit that can accomplish this is called a power combiner. An N-way combiner has one common port and N uncommon ports. The ports are places in the circuit that allow intentional connection to other external circuits. In general, a good power combiner should sum N input signals together with minimal signal energy or power loss. It should also keep the signals that are being summed together isolated from each other at the uncommon inputs. High uncommon port to uncommon port isolation can help to avoid impedance mismatches and signal degradation, and helps to minimize stress to circuitry connected to the uncommon ports.
A power combiner can sometimes be used for the purpose of splitting a single signal into two or more signals. When used in this configuration, the circuit is typically called a power splitter or power divider.
Additionally, sometimes special circuits are used to split signals or sample signals. Couplers, which may be thought of as a special case of a power splitter, are used to couple or sample some RF or microwave signal energy that may be used for various purposes. Often, it is desirable that a coupler have a coupled port and an isolated port, where the isolated port has little or none of the sampled signal, and the coupled port have a specified portion of the sampled signal. Some couplers are used to determine the direction of the sampled signal, whether it flows into the input port, or out of the input port. In these cases, the coupler directivity becomes an important parameter. Directivity is a function of the coupler isolation, again making it desirable for the coupler to have high isolation. In these cases, the coupler is often referred to as a Directional Coupler.
Power Combiners/Splittters
Before the 1960's, power combiner/splitter circuits tried to minimize insertion losses, but did not have significant port to port isolation. Circuits such as the hybrid T and quarter wave matched splitters were common. (U.S. Pat. No. 2,877,427—Butler and U.S. Pat. No. 2,963,664—Yeagly). Other combiner/splitter designs, (U.S. Pat. No. 3,529,265—Podell and ‘A New N-way Power Divider/Combiner Suitable for High Power Applications, Proc. Of 1975, IEEE MTT Seminar’—Gysel) demonstrate designs that exhibit isolation between uncommon ports but also are less sensitive to load mismatches and can accommodate high power signals.
U.S. Pat. No. 3,091,743, issued to Wilkinson, disclosed a power combiner/splitter that achieved relatively high port to port isolation between uncommon ports. The isolation was achieved by providing additional signal paths between the uncommon ports through resistors. The portion of a signal traveling through a resistor path had approximately the same amplitude as the signal that traveled to the common port and back to an adjacent uncommon port, but it was approximately 180 degrees out of phase, at the center frequency of design, having a zero degree phase shift at the center frequency of design. This created relatively high isolation between uncommon ports, due to the canceling effect of this circuit topology. For a single section design, however, port to port isolation greater than 40 or 50 dB between the uncommon ports, occurred only in a relatively small frequency bandwidth, on the order of 1 percent. This high isolation occurred at the center frequency of the design.
Cohn published a paper in 1968 (IEEE transactions on Microwave Theory and Techniques, Vol. MTT-16, No. 2, February 1968) where he introduced a multiple section power combiner/splitter. Cohn found that by adding additional quarter wave transmission line sections, plus an additional isolation resistor for each section, that high isolation between uncommon ports may be achieved over a broader bandwidth than a single section. A two section design can achieve up to 50 dB of isolation between uncommon ports over a 20 percent bandwidth, and up to 70 dB in a 5 percent bandwidth. While this design offered a better solution to many systems, this approach has the drawback of requiring additional circuit size due to additional transmission line sections. It also suffers from increased insertion loss, since the signal must travel through longer transmission lines. For many circuits, the relatively large size of a multisection power combiner cannot be tolerated. Additional insertion loss as well, is almost always a drawback in power combiners.
Since 1968, further advances of power combiners/splitters have been accomplished. Some designs exhibit improved power handling capability; others demonstrate decreased circuit size and others improve manufacturability. Some designs incorporate these improvements severally. Advances have also improved or modified electrical parameters of power combiners/splitters such as increased bandwidth, improved input and output VSWR and slightly lower insertion loss. However, to date there has been little improvement on increasing the magnitude of the in-band isolation between uncommon ports.
U.S. Pat. No. 5,489,880, issued to Swarup, discloses a power combiner/splitter that has increased port to port isolation for out-of-band signals. This is achieved by putting band pass filters at the uncommon ports. While this improves the out-of-band port to port isolation, the in-band isolation is not significantly improved. This design also suffers from increased insertion loss over a typical power combiner/splitter due to the signal going through a band pass filter as well as the combiner/splitter circuit.
Some systems require an uncommon port to port isolation that is higher than existing power combiner/splitters can provide. Placing circulators or isolators at uncommon ports, one can achieve isolations greater than 50 dB over relatively large bandwidths. These products, however, are relatively costly and exhibit non-linear characteristics at high input signal levels. They also increase the signal path insertion loss. Furthermore, they are not reciprocal circuits, and cannot be used as splitters without re-orienting the circulators or isolators.